Core prominence as a signature of restarted jet activity in the LOFAR radio-galaxy population

¹ Astronomy Department, Universidad de Concepción, Casilla 160-C, Concepción, Chile
² Joint Institute for VLBI ERIC, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
³ ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
⁴ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
⁵ INAF-Istituto di Radioastronomia, Via Gobetti 101, Bologna 40129, Italy
⁶ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via P. Gobetti 93/3, 40129 Bologna, Italy
⁷ Dipartimento di Fisica e Astronomia, Università di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
⁸ School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
⁹ Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712-1205, USA
¹⁰ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
¹¹ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK

^⋆ Corresponding author; dhanyagnair01@gmail.com

Received: 16 July 2024
Accepted: 23 August 2024

Abstract

Context. Recurrent phases of dormancy and activity occur in the supermassive black holes in active galactic nuclei. Characterizing the duty cycles of this process is crucial in understanding the impact of the energy released on the host galaxies, and their evolution. However, it is challenging to identify sources in the quiescent and restarted phases.

Aims. Our goal is to identify and characterize a substantial sample of radio galaxies in a restarted phase and explore the idea of core prominence as a signature of restarted activity. We expand our prior study of identifying restarted sources from a 30 deg² area in the Lockman Hole to a larger 424 deg² region in the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) extragalactic field using a sample of core-dominated radio galaxies selected visually.

Methods. We used the 144 MHz LOFAR survey images of the HETDEX field to identify galaxies with restarting jets. By assessing diverse selection criteria including radio core dominance along with low surface brightness extended emission, spectral index properties (e.g., steep or inverted spectra of a core and an ultra-steep spectrum of extended emission), and morphology, we found 69 candidate restarted radio galaxies in the HETDEX field.

Results. The restarted candidates show a diverse intrinsic morphology, spanning from FRI, FRII, core-with-halo, to asymmetric forms, feasibly proposing different progenitors. Within these 69 restarted candidates, we identified a subset of nine galaxies characterized by ultra-steep spectrum extended emission combined with high radio core prominence, representing previous and current epochs of jet activity. We interpret our findings for this small subset as support for a model in which the switch-on and switch-off mechanism happens with a relatively fast duty cycle in these sources. We found a peculiar case, J131728.61+561544.8, which appears to have altered its jet orientation, possibly due to changes in the angular momentum or spin of its supermassive black hole, interactions with surrounding material, and/or variations in the magnetic flux threading its accretion disk.

Conclusions. The restarted candidates span a range of radio luminosities from log₁₀(L_144 MHz/WHz⁻¹) = 23.24 to log₁₀(L_144 MHz/WHz⁻¹) = 26.80, and linear sizes between 88 and 1659 kpc at 144 MHz, with 16 sources identified as giant radio galaxies with sizes exceeding 0.7 Mpc. The total stellar content of restarted sources is consistent with massive elliptical galaxies, with at least 17% inhabiting cluster environments. Our findings at z < 0.4 suggest that many restarting radio galaxies are not found in rich cluster environments, aligning with the environmental properties of the broader radio-galaxy population. The present study confirms the core prominence as an effective parameter for selecting candidate restarted radio sources.